I don't know about the rest -- but for the 'caller' bit: caller() returns information about the sub that called the current sub. In the code that you gave, caller would normally return nothing when called as a regular script because nothing _called_ this code, instead this code is where everything starts.
What must be happening is that in PAR there is some script that kicks off, and then does a 'do' or similar on your script.
You might be able to illustrate this to yourself by dumping out a stack trace. Just above your 'unless', put:
use Carp qw( cluck );
That will print out the whole callchain. Now run your program directly, and then again from a PAR exe, and note the difference.
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